Electric current flow to a motor must be interrupted when the motor is overloaded in order to prevent the motor from suffering thermal damage. Digital techniques for controlling thyristors have been introduced, but the problem of providing a simple reliable overload interrupt using digital techniques and emulating the thermal characteristics of a motor remains unsolved. Past methods for protecting motors have used devices to detect motor current, and when the motor current exceeds a preset upper limit, the device turns off current flow to the motor. Other motor protection devices include binary alloy relays which are selected to have a thermal reponse characteristic similar to the thermal requirements of the motor. Further, melting alloy relays are employed for motor protection service, and melting allow relays also imitate the thermal response characteristics of a motor.
Operational amplifiers used to integrate a difference signal formed between a fixed value and a value proportional to an electric quantity have been used for overload protection, as disclosed by Gelfand, in U.S. Pat. No. 4,012,669, issued Mar. 15, 1977. Further digital sampling techniques have been used to indicate an overload condition, as disclosed by DePuy, U.S. Pat. No. 4,219,860, issued Aug. 26, 1980.
The problem of protection of an electric motor from an overload current situation by use of a simple digital technique remains unsolved.